U.S. patent number 5,612,489 [Application Number 08/601,627] was granted by the patent office on 1997-03-18 for enhanced sensitivity for oxygen and other interactive gases in sample gases using gas chromatography.
This patent grant is currently assigned to Air Products and Chemicals, Inc.. Invention is credited to Daniel J. Ragsdale, George H. Smudde, Jr., David A. Zatko.
United States Patent |
5,612,489 |
Ragsdale , et al. |
March 18, 1997 |
Enhanced sensitivity for oxygen and other interactive gases in
sample gases using gas chromatography
Abstract
The present invention is directed to a method of detecting trace
levels of contained interactive gas, such as oxygen, in gases
containing trace levels of interactive gas, such as oxygen, by
doping a known low level amount of interactive gas (i.e., oxygen)
into the detection apparatus to saturate interactive gas reactive
or adsorptive sites in the apparatus, thus allowing the accurate,
reproducible and responsive detection of contained interactive gas.
More particularly, the present invention dopes a carrier gas of a
gas chromatograph with low levels of oxygen in order to detect
contained trace oxygen in a sample of a gas being analyzed for
contained trace oxygen.
Inventors: |
Ragsdale; Daniel J.
(Quakertown, PA), Smudde, Jr.; George H. (Macungie, PA),
Zatko; David A. (Lansdale, PA) |
Assignee: |
Air Products and Chemicals,
Inc. (Allentown, PA)
|
Family
ID: |
24408184 |
Appl.
No.: |
08/601,627 |
Filed: |
February 14, 1996 |
Current U.S.
Class: |
73/23.35;
73/23.42; 422/89 |
Current CPC
Class: |
G01N
30/34 (20130101); G01N 2030/025 (20130101); G01N
30/64 (20130101); G01N 30/34 (20130101); G01N
30/66 (20130101); G01N 30/34 (20130101) |
Current International
Class: |
G01N
30/00 (20060101); G01N 30/34 (20060101); G01N
30/02 (20060101); G01N 30/66 (20060101); G01N
30/64 (20060101); G01N 030/34 () |
Field of
Search: |
;73/23.35,23.41,23.42,23.37,23.4,24.02,25.03 ;422/89 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Brock; Michael
Attorney, Agent or Firm: Chase; Geoffrey L.
Claims
We claim:
1. A method for detecting trace levels of an interactive gas
selected from the group consisting of oxygen, carbon monoxide,
hydrogen, carbon dioxide, fluorine, chlorine and water, contained
in a sample gas, which is mixed with a carrier gas, by detection of
said interactive gas using a gas chromatograph in gas communication
with a detector sensitive to said interactive gas, the improvement
comprising doping said carrier gas with a low level of said
interactive gas prior to said detection.
2. The method of claim 1 wherein said sample gas is selected from
the group consisting of hydrogen chloride, hydrogen bromide,
arsine, phosphine, silane, nitrogen trifluoride, hexafluoroethane,
trifluoromethane, nitrogen, argon, helium, hydrogen and mixtures
thereof.
3. The method of claim 1 wherein said carrier gas is selected from
the group consisting of helium, argon, nitrogen and mixtures
thereof.
4. The method of claim 1 wherein said trace level of interactive
gas in said sample gas is less than 1000 ppm.
5. The method of claim 1 wherein said trace level of interactive
gas in said sample gas is less than 100 ppm.
6. The method of claim 1 wherein said trace level of interactive
gas in said sample gas is less than 1 ppm.
7. The method of claim 1 wherein said low level of interactive gas
which is doped into said carrier gas is less than 10 ppm.
8. The method of claim 1 wherein said low level of interactive gas
which is doped into said carrier gas is less than 1 ppm.
9. The method of claim 1 wherein said low level of interactive gas
which is doped into said carrier gas is less than 100 ppb.
10. The method of claim 1 wherein said low level of interactive gas
is doped into said carrier gas by the method selected from the
group consisting of dynamic dilution, permeation and calibrated
leak.
11. The method of claim 1 wherein said detector sensitive to said
interactive gas is selected from the group consisting of a
thermoconductivity detector, a discharge ionization detector, a
helium ionization detector, and a high frequency discharge
detector.
12. The method of claim 1 wherein said gas chromatograph is packed
with an adsorbent selected from the group consisting of zeolitic
molecular sieves, porous polymers, silica gel, carbon molecular
sieves and mixtures thereof.
13. The method of claim 1 wherein said low level of interactive gas
is doped into said carrier gas downstream of a point of
introduction of said sample gas into said carrier gas.
14. The method of claim 1 wherein said low level of interactive gas
is doped into said carrier gas upstream of a point of introduction
of said sample gas into said carrier gas.
15. A method for detecting trace levels of oxygen in a sample gas,
which is mixed with a carrier gas, by detection of said oxygen
using a gas chromatograph in gas communication with a detector
sensitive to oxygen, the improvement comprising doping said carrier
gas with a low level of oxygen prior to said detection.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention is directed to a method of detecting trace
levels of contained interactive gases, such as oxygen, in gases
containing trace levels of interactive gas by doping a known amount
of the interactive gas into the detection apparatus to saturate
interactive gas reactive or adsorptive sites in the apparatus, thus
allowing the accurate and responsive detection of the contained
interactive gas. More particularly, the present invention dopes a
carrier gas of a gas chromatograph with low levels of oxygen in
order to detect contained trace oxygen in a sample of a gas being
analyzed for contained trace oxygen.
BACKGROUND OF THE INVENTION
The industrial gas industry is faced with ever more stringent
requirements for purity in industrial gases for research and for
industries, such as the electronic fabrication industry.
Oxygen is one of the contaminant gases for which tight
specification requirements have been set by such industries,
particularly for inert gases used for blanketing or prevention of
oxidation. Oxygen impurity levels must be very low in such gases
which are used for inerting atmospheres. Typically, those gases
include nitrogen and argon. Other gases which face similar
specifications include carbon monoxide, hydrogen, carbon dioxide,
fluorine, chlorine and water.
In the purification, storage and dispensing of industrial gases, it
is necessary to check or monitor purity on a batch or continuous
basis. When the levels of impurities in gases is very low, such as
parts per million (ppm) or parts per billion (ppb), and the
monitoring of the gases for impurities is done in a batch or
non-continuous basis, difficulties arise in having rapid,
reproducible, accurate results. Effectively, it is difficult to
reach a steady state condition.
Although it is possible to at least partially overcome this problem
by using a dedicated analyzer, when the detection requires a
sophisticated and/or expensive detection device, such as a gas
chromatograph, the trend is to use such detection equipment for a
plurality of analyses, so that it is impossible or impractical to
reach a steady state for any particular impurity analysis.
The art has recognized that the adsorbent used in gas
chromatographs for oxygen analysis can have improved performance if
initially subjected to a singular oxidation treatment. This is
reported in U.S. Pat. No. 4,744,805 and 4,713,362.
Removal of oxygen from bulk gases, in contrast to analysis, is
described in U.S. Pat. No. 4,747,854.
The prior art has failed to achieve a solution to the problem of
analysis of trace levels of interactive gases, such as oxygen, in
gases requiring high purity specifications. The present invention
provides an inexpensive method for trace level interactive gas,
(i.e., oxygen,) impurity detection in gases using gas
chromatography that is particularly valuable in non-steady state
situations. The method is fast, accurate and sensitive to
interactive gas, (i.e., oxygen), levels in the ppm and ppb range.
This overcomes a problem that has existed for years that has
prevented reproducible analysis.
BRIEF SUMMARY OF THE INVENTION
The present invention is a method for detecting trace levels of an
interactive gas selected from the group consisting of oxygen,
carbon monoxide, hydrogen, carbon dioxide, fluorine, chlorine and
water, contained in a sample gas, which is mixed with a carrier
gas, by detection of the interactive gas using a gas chromatograph
in gas communication with a detector sensitive to the interactive
gas, the improvement comprising doping the carrier gas with a low
level of the interactive gas upstream of the detection.
Preferably, the sample gas is selected from the group consisting of
hydrogen chloride, hydrogen bromide, arsine, phosphine, silane,
nitrogen trifluoride, hexafluoroethane, trifluoromethane, nitrogen,
argon, helium, hydrogen and mixtures thereof.
Preferably, the carrier gas is selected from the group consisting
of helium, argon, nitrogen and mixtures thereof.
Preferably, the trace level of interactive gas in the sample gas is
less than 1000 ppm.
More preferably, the trace level of interactive gas in the sample
gas is less than 100 ppm.
Most preferably, the trace level of interactive gas in the sample
gas is less than 1 ppm.
Preferably, the low level of interactive gas which is doped into
the carrier gas results in less than 10 ppm interactive gas in the
carrier gas.
More preferably, the low level of interactive gas which is doped
into the carrier gas results in less than 1 ppm interactive gas in
the carrier gas.
Most preferably, the low level of interactive gas which is doped
into the carrier gas results in less than 100 ppb interactive
gas.
Preferably, the low level of interactive gas is doped into the
carrier gas by the method selected from the group consisting of
dynamic dilution, permeation and calibrated leak.
Preferably, the detector sensitive to the interactive gas is
selected from the group consisting of a thermoconductivity
detector, a discharge ionization detector, a helium ionization
detector, and a high frequency discharge detector.
Preferably, the gas chromatograph is packed with an adsorbent
selected from the group consisting of zeolitic molecular sieves,
porous polymers, silica gel, carbon molecular sieves and mixtures
thereof.
Preferably, the low level of interactive gas is doped into the
carrier gas upstream of a point of introduction of the sample gas
into the carrier gas.
Alternatively, the low level of interactive gas is doped into the
carrier gas downstream of a point of introduction of the sample gas
into the carrier gas.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a gas chromatogram of a sample gas containing nitrogen
and 3.9 ppm of oxygen in a helium carrier gas under the conditions
of Example 1. Five separate injections are shown. Detected oxygen
increases with subsequent injections.
FIG. 2 is a gas chromatogram of a sample gas containing 2.4 ppm of
oxygen in the sample recited in Example 2 in a helium carrier gas.
One injection is shown because all injections resulted in the same
oxygen response. Oxygen remains undetected.
FIG. 3 is a gas chromatogram of the sample gas of Example 3
containing 5.8 ppm of oxygen in a helium carrier gas and using low
level oxygen doping of that carrier gas. One injection is
shown.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have determined that by doping the carrier
gas to a gas chromatograph with interactive gas, such as oxygen, at
a low level, typically less than 10 ppm (parts per million by
volume), the detection of trace levels of the interactive gas, such
as oxygen impurity in sample gases, such as bulk or specialty
gases, can be improved.
Oxygen is the interactive gas that is most significant for trace
detection in gases for the electronic industry, but other
interactive gases, such as carbon monoxide, hydrogen, carbon
dioxide, fluorine, chlorine and water can be analyzed by the
present invention. The use of the term interactive is to indicate a
gas that is reactive with or sorbed on the materials of
construction of passageways, valves, and other equipment or column
media (i.e., adsorbent) of the devices used for detection of the
interactive gas to the point that the gas at the trace levels in
which it is present in the sample gas is significantly or totally
removed from the sample gas prior to detection so as to
significantly alter or preclude accurate detection of the gas in
the sample gas. Water is included in the interactive gases because
of its ability to be present in trace quantities in the sample gas
in the form of water vapor. When hydrogen is the sample gas,
hydrogen would not be the trace interactive gas.
Typically industrial gases provided to demanding end uses, such as
research or the electronics fabrication industry, have
specifications which tolerate only very low levels of impurities or
contaminants. It is difficult to detect impurities at the levels
required by such end users. Oxygen is particularly difficult to
detect at trace levels in other gases, because the present
inventors have ascertained that low levels of oxygen are sorbed or
chemically bound to various equipment surfaces so as not to be
detected by the appropriate detector sensitive to oxygen, such as
in a gas chromatograph. This is particularly problematic when the
gas sampling ports and lines from the industrial gas source to be
analyzed to the gas chromatograph are lengthy or convoluted. It is
also problematic within the passageways of the gas chromatograph
and the passageways leading from the gas chromatograph to the
detector. Other interactive gases present the same problems.
Oxygen, particularly at trace levels less than 1000 ppm, can
significantly or fully sorb on column packing, metal surfaces,
plastics, Teflon fittings, etc. This can scavenge the oxygen
impurity from the sample gas before it can be detected at the
detector of a gas chromatograph. Various attempts have been made by
the present inventors to use oxygen compatible materials of
construction and highly finished materials to diminish oxygen
sorption or reaction with such surfaces. The present inventors have
also attempted to design sampling passageways, gas chromatograph
hookups and gas chromatograph detector arrangements so as to
diminish oxygen consumption (adsorption or reaction) in the gas
chromatograph. They have also attempted to minimize areas where
oxygen can hang-up, such as in eddy currents, deadspaces, etc.
These attempts have typically resulted in inconsistent and
temporary improvements in trace oxygen detection, and the speed and
accuracy of trace oxygen detection, particularly in non-continuous
sampling for trace oxygen. It is known that other interactive gases
behave similarly.
Unexpectedly, the present inventors have found that by doping the
carrier gas carrying the sample gas which is to be analyzed for
trace oxygen with a known low level quantity (typically <10 ppm)
of doping oxygen in the carrier gas, the detection limit for the
trace oxygen impurity is greatly improved, along with the response
time to achieve the detection. Potentially, the oxygen doping of
the carrier gas can be at a level comparable to or less than the
expected trace oxygen in the sample gas to be analyzed. This
heightened sensitivity of the gas chromatograph and detector
sensitive to trace oxygen impurity in a sample gas is contrary to
what one would expect. For instance, one might expect that for
trace levels of oxygen, adding oxygen might adversely interfere
with the signal for the trace oxygen. Also, it is important to be
able to closely regulate the doping rate. Uncontrolled or
inconsistent doping of oxygen into the carrier gas will not result
in greater sensitivity in gas chromatograph trace oxygen detection
at very low trace oxygen levels. Consistent doping of oxygen into
the carrier gas can be performed by dynamic dilution, permeation or
calibrated leak or any other method which allows for precise, low
level oxygen doping (typically <10 ppm, but potentially at the
level of trace oxygen in the sample to be analyzed). Doping by
dynamic dilution is performed by consistent mixing of the carrier
gas with a stream containing either pure oxygen or a mixture of
oxygen and the same gas as the carrier gas. Doping by permeation is
performed by consistently diffusing oxygen from a separate
reservoir through a material into the carrier gas. Doping using a
calibrated leak is performed by constantly injecting a quantity of
oxygen through controlled leak typically consisting of an orifice
or capillary tube into the carrier gas. It is contemplated that
other interactive gases can be handled in a similar manner.
Alternatively, doping could be performed by using a mixture of
carrier gas and the doping gas dispensed from an industrial gas
cylinder.
Typically, it is appropriate to dope the interactive gas (i.e.,
oxygen) into the carrier gas upstream of the gas sampling valve or
syringe injection point (point of introduction of sample gas) in a
gas chromatograph. Alternatively, the interactive gas (i.e.,
oxygen) can be doped into the carrier gas downstream of the gas
sampling valve (point of introduction). Downstream of the gas
sampling valve is where non-continuous gas flow occurs, which can
be problematic for interactive gas (i.e., oxygen) analysis. Doping
can occur internal to the gas chromatograph, in a separate module
attached to the gas chromatograph or in a separate module detached
from the gas chromatograph.
The detector sensitive to the interactive gas (i.e., oxygen) is
selected from the group consisting of a thermoconductivity
detector, a discharge ionization detector, a helium ionization
detector, and a high frequency discharge detector.
Although not wanting to be held to any particular theory, the
present inventors believe that the doping of the carrier gas to the
gas chromatograph with a calibrated, constant, low level (typically
<10 ppm) of interactive gas (i.e., oxygen), allows the
interactive gas (i.e., oxygen) to saturate or react with any
sorptive or reactive sites in the passageways leading from the
point of introduction of the carrier gas to the sample downstream
to the detector sensitive to the interactive gas (i.e., oxygen).
Therefore, the doping interactive gas (i.e., oxygen) eliminates the
opportunity for the trace interactive gas (i.e., oxygen) from the
sample gas being sorbed or reacted and provides a baseline
background of interactive gas (i.e., oxygen) detection against
which the spikes of the actual trace interactive gas (i.e.,
oxygen)in the sample gas being analyzed can be detected accurately
and with fast response from when the sampling is actually
performed.
Sample gases which can be analyzed for trace interactive gas such
as oxygen by this method include: the bulk industrial gases;
nitrogen, argon, helium and hydrogen, and the specialty industrial
gases; hydrogen chloride, hydrogen bromide, arsine, phosphine,
silane, nitrogen trifluoride, hexafluoroethane,
trifluoromethane
The doping interactive gas (i.e., oxygen) is preferably 99.999% by
volume. Most preferably, the doping interactive gas (i.e., oxygen)
is 99.9999% by volume. Alternatively, the low level of interactive
gas (i.e., oxygen) in the carrier gas can be provided by a more
dilute mixture of the interactive gas (i.e., oxygen) in the same
gas as the carrier gas or other industrial gas.
The amount of doping interactive gas (i.e., oxygen) injected into
the carrier gas is less than 10 ppm of interactive gas (i.e.,
oxygen) in the resulting doped carrier gas. Preferably, the range
is less than 1 ppm. More preferably, the range is less than 100
ppb.
The carrier gas can be selected from the group comprising helium,
argon and nitrogen.
This technique permits reproducible detection of trace interactive
gas (i.e., oxygen) at quantities less than 1000 ppm. More
preferably, it is useful to detect trace interactive gas (i.e.,
oxygen) of less than 100 ppm. Most preferably, it is useful in the
range of less than 1 ppm. The values of ppm and ppb (parts per
billion) are on a volumetric basis.
EXAMPLE 1
A sample containing 3.9 ppm O.sub.2 and an undetermined amount of
N.sub.2 in He was analyzed in a gas chromatograph without using low
level oxygen doping of the carrier gas (He) in a series of 5 sample
injections within 15 minutes. The gas chromatograph was a Gow-Mac
590 with a 10 foot molecular sieve 5A analytical column and a
discharge ionization detector (DID). The discharge current was 8.03
mA @ 600 v, the column flow rate was 31 standard cubic centimeters
per minute (sccm) and the detector flow rate was 12 sccm. The
sample size was 1 ml and the amplifier range was 10.sup.-12 with an
attenuation of 1. FIG. 1 reports the results of the analysis and
demonstrates the case where O.sub.2 from the sample is not
completely consumed by oxygen scavenging sites of the gas
chromatograph. In this case, the peak area and height for O.sub.2
is lowest after a period of time where the instrument has not
analyzed a sample containing O.sub.2. In subsequent injections, the
O.sub.2 peak areas and heights increase. The O.sub.2 peak areas and
heights from repeated injections attained constant values after 5
or more injections. This illustrates the added time required to
reach a steady state so that accurate and consistent results can be
obtained in a technique that does not use the methodology of the
present invention.
EXAMPLE 2
A sample containing 1.9 ppm H.sub.2, 2.4 ppm O.sub.2, 1.9 ppm
N.sub.2, 2.0 ppm CH.sub.4, 2.1 ppm CO in He was subjected to
analysis in a gas chromatograph without doping of the carrier gas
with low levels of oxygen. The gas chromatograph was a Gow-Mac 590
with a 6 foot molecular sieve 5A analytical column and a discharge
ionization detector (DID). The column flow rate was 29 sccm and the
detector flow rate was 11 sccm. The sample size was 1 ml and the
amplifier range was 10.sup.-12 with an attenuation of 1. The gas
chromatograph analysis of oxygen in the sample gas contained in the
He carrier gas is illustrated in FIG. 2 for one of several
injections of the sample into the gas chromatograph. In this
instance, the sample containing 2.4 ppm O.sub.2 gave no appreciable
peak, even after repeated injections over a period of time.
Apparently, almost all O.sub.2 is consumed by the gas chromatograph
system, and subsequent injections did not increase the peak area or
height. Quantification of trace O.sub.2 under these conditions is
not possible.
EXAMPLE 3
An experiment was conducted using the procedure outlined in the
present invention of low level oxygen doping of the sample's
carrier gas. The sample contained 5.3 ppm H.sub.2, 5.8 ppm O.sub.2,
6.3 ppm N.sub.2, 4.9 ppm CH.sub.4, and 5.1 ppm CO in He. Table 1
shows the reported peak areas for the seven injections of the
sample gas doped with 5.8 ppm O.sub.2 in the He carrier gas after
the gas chromatograph instrument was idle overnight. Each oxygen
analysis is very consistent with the other analyses in the
experiment. FIG. 3 shows the resulting chromatogram for one of the
analyses. The peak area for O.sub.2 is large and results are
consistent for multiple analyses. These data were obtained on the
same gas chromatograph as Examples 1 and 2. The column flow rate
was 32 sccm and the detector flow rate was 15 sccm. The analytical
column was a 3 foot molecular sieve 13X and the precolumn was 3 ft
silica gel. The sample size was 1 ml and the amplifier range was
10.sup.-12 with an attenuation of 1. This example demonstrates that
when the present invention is implemented, high consistent O.sub.2
sensitivity is attained.
TABLE 1 ______________________________________ O.sub.2 Peak Area
(mA .multidot. sec) ______________________________________ 1057
1056 1054 1056 1056 1059 1056
______________________________________
EXAMPLE 4
A 1 ml sample of helium containing oxygen at a flow rate of 100
sccm was further used to test the method of oxygen doping. Oxygen
was doped into a helium carrier gas of a gas chromatograph at a low
level of 26 parts per billion (ppb) of oxygen in the carrier gas.
The oxygen was doped by using a Kin-Tek Laboratories permeation
cell. The cell was maintained at room temperature during the doping
and the oxygen content of the carrier gas was determined by the
partial pressure of the oxygen in the cell. The oxygen level was
confirmed with a Delta F Nanotrace oxygen analyzer. The detector
was a discharge ionization detector and the discharge current was
8.0 mA at 600 V. The oxygen doped helium carrier gas was passed
through a 13X molecular sieve analytical column at 32.degree. C. at
a flowrate of 30 cc/min. The lowest measured value for oxygen was
23 ppb and the limit of detection was 17 ppb. Under the same
conditions, the limits of detection for hydrogen, nitrogen, methane
and carbon monoxide were 12, 24, 6, and 40 ppb, respectively.
As can be seen from the above example, the method of the present
invention is a significant improvement in the art of detecting
trace interactive gas, preferably oxygen, in sampled gases
containing interactive gas (i.e., oxygen) in trace quantities. The
method of the present invention provides fast, sensitive and
reproducible detection of interactive gas (i.e., oxygen) at trace
levels below 1000 ppm.
The prior art has attempted to overcome the problem of trace
interactive gas (i.e., oxygen) detection by designing systems with
less interactive gas (i.e., oxygen) receptive surfaces, by
designing systems with less travel distance or by using repetitious
detection samplings. These attempts have not been successful in
achieving a sensitive, quick, low level interactive gas (i.e.,
oxygen) detection methodology, particularly for batch or
non-continuous sampling. The present invention overcomes the
drawbacks of the prior art by providing interactive gas, preferably
oxygen, doping of the carrier gas of a gas chromatograph to
successfully achieve sensitive, rapid, low level detection of trace
interactive gas, preferably oxygen, in sampled gases, even in batch
or non-continuous samplings.
The present invention has been described with regard to several
preferred embodiments, however the scope of the present invention
should be ascertained from the claims which follow.
* * * * *